Allow overriding MacOS.version.

Library/Homebrew/cask/lib/hbc/cli.rb

@@ -142,6 +142,10 @@ module Hbc
     end
 
     def self.process(arguments)
+      unless ENV["MACOS_VERSION"].nil?
+        MacOS.full_version = ENV["MACOS_VERSION"]
+      end
+
       command_string, *rest = *arguments
       rest = process_options(rest)
       command = Hbc.help ? "help" : lookup_command(command_string)

Library/Homebrew/os/mac.rb

@@ -27,6 +27,11 @@ module OS
       @full_version ||= Version.new((ENV["HOMEBREW_MACOS_VERSION"] || ENV["HOMEBREW_OSX_VERSION"]).chomp)
     end
 
+    def full_version=(version)
+      @full_version = Version.new(version.chomp)
+      @version = nil
+    end
+
     def prerelease?
       # TODO: bump version when new OS is released
       version >= "10.13"